When I was at Guadalupe I shot with a 5D (full-frame sensor) and a 16-35mm lens. It was rare that the animals were so close that I couldn't frame the shots well. When I was down 30 feet in the submersible cage the sharks were often too far away to shoot with the 16-35. All that being said, if I were to do it again I would shoot with my 16-35, but I would also bring my 15mm fisheye just for a different style of shot.
Here are my photos so you can get an idea of how that lens was shooting out there: (but please note that I was new to u/w photography at the time and those shots aren't great).
Thanks - Gina - that's what I hoped to hear:) Great shots!